Solitary bee larvae prioritize carbohydrate over protein in parentally provided pollen

1. Most organisms must regulate their nutritional intake in an environment full of complex food choices. While this process is well-understood for self-sufficient organisms, dependent offspring, such as bee larvae, in practice have limited food choices because food is provided by parents. Nutrient balancing may therefore be achieved by offspring, by parents on offspring's behalf, or by both, whether cooperatively or in conflict. 2. We used the Geometric Framework to investigate the capacity of dependent larval mason bees Osmia bicornis to regulate their intake of protein and carbohydrate, as well as these nutrients' effects upon growth and survival. Female Osmia seal eggs individually inside cells they have provisioned with pollen, and have no contact with developing offspring, allowing offspring choices to be studied in isolation. Herbivorous insect larvae are typically expected to balance protein and carbohydrate to maximize growth and reproduction. 3. Contrary to prediction, carbohydrate and not protein mediated both growth and survival to pupation. Accordingly, larvae prioritized maintaining a constant intake of carbohydrate and self-selected a relatively carbohydrate-biased diet compared to other hymenopterans, while tolerating wide excesses and deficiencies of protein, rendering them potentially vulnerable to dietary change or manipulation. Reasons for prioritizing carbohydrate may include (a) the relative abundance of protein in their normal pollen diet, (b) the relative paucity of nectar in parental provisions making carbohydrate a scarce resource or (c) the requirement for diapause for all O. bicornis larvae. Larvae were intolerant of moderate dietary dilution, likely reflecting an evolutionary history of nutrient-dense food. 4. Our results demonstrate that dependent offspring can remain active participants in balancing their own nutrients even when sedentary, and, moreover, even in mass provisioning systems where parents and offspring have no physical contact. Research should now focus on whether and how evolutionary interests of parent and dependent offspring coincide or conflict with respect to food composition, and the implications for species' resilience to changing environments.